The present invention relates to a bridge apparatus connecting between LANs (Local Area Networks) and a communication system between networks using the apparatus, and more specifically to a bridge apparatus connecting between a LAN transferring in fixed data unit and a control method thereof.
In a backbone LAN incorporating a frontend LAN, a routing system and a control method thereof in the prior art is disclosed in Japanese patent application laid-open No. 196132/1988 and performs as follows.
In communication between stations connected to frontend LANs, each station need not know the routing information regarding frontend LAN-ID to which a destination station is connected, nor its node address of a backbone LAN. A sending station address and a destination station address are added to the transmission data. If the destination address is in the same LAN as that of the sending station, the destination station can receive this data directly. Also if the destination station is connected through the backbone LAN to another frontend LAN, communication can be performed with transmittivity as if the transmitting and receiving stations were in the same LAN. This routing control is performed basically in accordance with the algorithm of learning bridge specified in the IEEE 802.1d MAC (Media Access Control) Bridges by nodes constituting LANs as follows.
A node holds an entry table where positions of stations are registered. When the node performs retrieval from the entry table regarding a destination station address of the frame supplied by a frontend LAN, and if the destination station is in the frontend LAN connected to the self node, the received frame is discarded. If the destination station is not in the frontend LAN connected to the self node or if it cannot be found, the received frame is transmitted by broadcast to the backbone LAN. Each backbone LAN node receives this frame and performs retrieval of the destination address from the entry table. If the destination station address is in the frontend LAN incorporated by the self node, the received frame is forwarded to the frontend LAN incorporated by the self node allowing the destination station to receives this frame. If the entry cannot be found, the frame is forwarded to the frontend LAN. If the destination station is in a frontend LAN incorporated by another node, the received frame is discarded.
As above described, the entry table, deciding whether a frame is forwarded using a station address as a key, has information indicating whether the station having the address is at the frontend LAN side or at the backbone LAN side. The entry table possessed by each node can learn whether a station is at the frontend LAN side or at the backbone LAN side from the sending (source) station address of frames flowing through the frontend LAN or frames flowing through the backbone LAN.
In the above-mentioned prior art, however, forward processing between frontend LANs through a backbone LAN constituted by a plurality of physical or logical links is not considered. Data transfer from the backbone LAN to the frontend LAN cannot be performed.
Also in the prior art, forwarding between a plurality of frontend LANs incorporated in a node is not considered and therefore data transfer cannot be performed to these frontend LANs.
In the prior art, since learning of the routing information in a backbone LAN in fixed length data unit is not considered, following point becomes a problem.
That is, data received from the frontend LAN by the node of the backbone LAN is divided in a plurality of fixed length data units. However, only one of the data units of the backbone LAN include the routing information, and other nodes of the backbone LAN does not recognize data to be learned.
Also in the prior art, since learning the routing information of a backbone LAN constituted by a plurality of physical or logical links is not considered, the following point becomes a problem.
When the node receiving the frame of the frontend LAN performs transmission by broadcast to the backbone LAN, the same data is transmitted along all links. On the other hand, as the learning manner in other node, the learning must be performed from all links. In this case, the learning by the same information occurs in the number of the links, and the futile learning process increases.
Also in the prior art, since learning of the routing information of a network constituted by a loop-shaped link is not considered, following point becomes a problem.
That is, the node forwarding data again receives the data after taking a round of the loop. Consequently, the learning is performed twice, i.e., when the data is received from the frontend LAN and when the data is received from the backbone LAN, and the futile learning process increases.
In addition, the prior art relating to a bridge circuit is disclosed, for example, in U.S. Pat. No. 4,597,078, "Bridge circuit for interconnecting networks".